Abstract: To produce a desirable amount of desirable radical and/or ion in treating a substrate such as etching the substrate, depositing a thin film on the substrate and the like by using plasma and the like. As a treating gas, a gas such as CFmIn and the like containing both a strongly bonded halogen element (F etc.) and weakly bonded halogen element (I etc.) is used. A substrate is treated by active species produced by exciting the treating gas by an excitation means capable of providing an energy which cannot dissociate the strong bond but can dissociate the weak bond. Preferable excitation means is capable of emitting a monochromatic irradiation, having a single value of excitation energy, such as electron beam, light etc., or otherwise capable of providing plasma, having a peak energy value of electrons and sharp electron energy distribution, such as UHF plasma etc.